What is the length of time for a pituitary adenoma to grow 2 cm?

It depends? There are many kind of adenoma, some are functional and some are not, it is very hard to guess.
This varies. The length of time that it takes a pituitary adenoma to grow to 2 cm varies depending on the type of tumor and whether or not it is hormonally active and has had previous operations. Typically, the majority are benign and slow growing. It may take months to years to reach 2 cm(again depends on the starting size). The best way to get an idea of the growth rate is to do serial MRI scans to follow.